Recently, composite materials using a carbon fiber as a reinforced fiber have been frequently used as structural materials of aircraft, etc. due to their excellent mechanical characteristics such as lightness and high strength. These composite materials are molded, for example, from a prepreg, which is an intermediate product, produced by impregnating a reinforced fiber with a matrix resin through molding and processing steps including heating and pressurizing. As such, it is required that optimal materials or molding and processing means for them are adopted for obtaining a desired composite material. In addition, depending on applications, the carbon fiber that is a reinforced fiber may require still higher strength, etc. For example, for lightening of a composite material for aircraft, although elasticity should be increased while maintaining the strength of the carbon fiber, carbon fibers are generally increased in brittleness and decreased in elongation as the elastic modulus is increased, whereby it is difficult to obtain a composite material having high composite performance.
In the aircraft field, carbon fibers with medium strength and elastic modulus, for example, carbon fibers with a strength of about 5,680 MPa and an elastic modulus of about 294 GPa have been conventionally used. However, recently, mainly for lightening of the airframe, composite materials having still higher performance have been required and in response to this carbon fibers having both high strength and high elasticity have been attempted to be developed. However, the elastic modulus and elongation are in trade-off relationship, so that carbon fibers are lowered in elongation and increased in brittleness as the elastic modulus is increased. Hence, it has been extremely difficult to produce a high performance carbon fiber having both high elasticity and high strength as well as hardly lowered physical properties such as brittleness. In particular, this tendency becomes remarkable when the elastic modulus exceeds 294 GPa, whereby the development has been extremely difficult including securement of stable physical properties.
In making the carbon fiber and the matrix resin composite, it is essential to improve also strength, elastic modulus, etc. of the carbon fiber itself as described above to pursue high performance. In addition, the improvement of the intensity and elastic modulus, etc. of the carbon fiber have been conventionally discussed in different ways. In particular, the improvement and modification of a pre-oxidation step and/or carbonization (including graphitization) step for producing carbon fibers from polyacrylic precursor fibers have been aggressively studied even comparatively recently (see, e.g., Patent Documents 1 to 5). However, no industrially advantageous method has been necessarily established of producing a carbon fiber with high strength and high elasticity suitable for a composite material that requires present, particularly high composite performance.    Patent Document 1: Japanese Laid-Open Patent Application No. 5-214614    Patent Document 2: Japanese Laid-Open Patent Application No. 10-25627    Patent Document 3: Japanese Laid-Open Patent Application No. 2001-131833    Patent Document 4: Japanese Laid-Open Patent Application No. 2003-138434    Patent Document 5: Japanese Laid-Open Patent Application No. 2003-138435
In general, as a method for producing a carbon fiber using a polyacrylic precursor fiber is known a method of production that includes oxidizing (fireproof treating) a precursor fiber while drawing or shrinking the precursor fiber at 200 to 280° C. in an oxidation atmosphere and then carbonizing the resultant material at 300° C. or higher in an inert-gas atmosphere. In particular, the method of treating a fiber in the pre-oxidation step greatly affects the strength development of a carbon fiber, and has long been studied in a variety of manners.
Reports have long been made, for example, on obtaining a high strength carbon fiber by carbonizing a pre-oxidation thread having a fiber density of 1.30 to 1.42 g/cm3, produced in a pre-oxidation step in the elongation rate range of −10 to +10% (an elongation rate of 0.9 to 1.1) (see, for example, Patent Document 6), obtaining a high-strength carbon fiber by giving an elongation rate of 3% or more (a draw ratio of 1.03 or more) until the fiber density reaches 1.22 g/cm3, substantially suppressing a subsequent shrinkage and subjecting the resulting fiber to pre-oxidation, and then carbonizing (see Patent Document 7), or obtaining a carbon fiber having a strand strength of 460 kgf/mm2 or more by subjecting a fiber to pre-oxidation with an elongation rate of 3% or more (a draw ratio of 1.03 or more) and further to drawing treatment with an elongation rate of 1% or more (a draw ratio of 1.01 or more) until the fiber density reaches 1.22 g/cm3, and then carbonizing (see Patent Document 8).    Patent Document 6: Japanese Published Examined Application No. 63-28132    Patent Document 7: Japanese Published Examined Application No. 3-23649    Patent Document 8: Japanese Published Examined Application No. 3-23650